Deformation and recrystallization mechanisms in naturally deformed omphacites from the Sesia-Lanzo zone; geophysical consequences

Abstract

Naturally deformed omphacites (clinopyroxenes) from the Sesia-Lanzo zone, western Italian Alps have been studied using light and transmission electron microscopy techniques. Deformation and recrystallization of omphacite took place at 550–600°C and minimum pressures of 14–16 kbar. TEM reveals isolated free dislocations, loops, nodal points, tiltwalls, complex dislocation networks, minor twins, stacking faults and antiphase domain boundaries. The activated slip systems are 1 2 〈110〉{110}, [001]{110} and [001](100) assisted by climb. In nodal points and complex dislocation networks dislocations with Burgers vector 1 2 〈112〉 are present. In low-strain domains most of the subgrain, transitional and new grain boundaries subparallel to [001] develop from 〈110〉 tiltwalls which accommodate lattice bending produced by the 1 2 〈110〉{110} slip system. A component of the misorientation across such newly formed (sub)grain boundaries is, however, also formed by (sub)grain-boundary migration. Grain rotations in the intermediate- (and high-) strain domains are more complex, but can be explained by the activated slip systems. The omphacite microstructures indicate that initial strain-induced recrystallisation occurred by a rotation mechanism where single crystals become polycrystals by the development of numerous high-angle grain boundaries. At a subsequent more advanced stage migration recrystallization is also activated and a different kind of recrystallization mechanism occurs which involves the discontinuous development of new strain-free grains. These grains are derived from the extremum of the orientations produced by heterogeneous strain and slip system activity. The evident ductility of omphacite during low- to intermediate-eclogite metamorphism is inferred to be facilitated by hydrolitic weakening effects. As such the plastic behaviour of omphacite should be incorporated in geophysical flow models dealing with subducted oceanic and/or lithosperic plate margins.